The present invention relates to the recovery of uranium from subterranean ore deposits and more particularly to an in-situ leaching operation in which an aqueous lixiviant containing sulfuric acid and carbon dioxide is preceded by a sulfuric acid solution which is substantially free of carbon dioxide.
In an in-situ leaching operation, a lixiviant is introduced into a subterranean uranium ore deposit through a suitable injection system. The lixiviant may be an acidic or alkaline medium which solubilizes uranium values as it traverses the ore body. The pregnant lixiviant is then withdrawn from the ore body through a production system and treated to recover uranium therefrom by suitable techniques such as solvent extraction, direct precipitation, or by adsorption and elution employing an ion exchange resin.
In acid leaching operations, the most commonly employed acid is sulfuric acid. The sulfuric acid normally is present in the lixiviant in a concentration to provide a pH of 2 or less. Normally, sufficient acid is present in the lixiviant to provide an excess of acid over that consumed by uranium solubilization and inorganic carbonates within the formation in order to retain a relatively low pH in the pregnant lixiviant as it is withdrawn from the ore. For example, as disclosed in Merritt, R. C., THE EXTRACTIVE METALLURGY OF URANIUM, Colorado School of Mines, Research Institute, USA (1971), at pages 62 and 63, under the heading "Acid Concentration", it is desirable to provide excess acid in order to prevent reprecipitation of uranium. Thus, Merritt discloses that free acid concentrations of from 1 to 90 grams per liter (0.1 to 9 weight percent) may be required continuously during the contact period.
The presence of carbonate materials in subterranean rock deposits containing uranium limits the use of acid lixiviants not only with respect to acid consumption by the carbonates but also due to the precipitation of reaction products such as calcium sulfate which may result in plugging of the formation. For example, Merritt at pages 109-111 describes an in-situ leaching procedure employing sulfuric acid which utilizes the natural flow of ground water through the subterranean ore deposit as a containment shell for the leaching solution. Merritt states that when lime bearing materials (carbonates) are present, the usual practice is to start feeding the solution a low concentration of from 1.0 to 1.5 grams of H.sub.2 SO.sub.4 per liter until acid is detected in the produced effluent. At this time, the lime in the formation is considered to have been neutralized and the acid concentration is increased in increments to a maximum of 5 grams per liter.
The use of sulfuric acid in in-situ leaching of uranium is also disclosed in U.S. Pat. No. 3,309,141 to Fitch et al. In this process, sufficient sulfuric acid is used to react with all the acid consuming materials in the ore body and yet leave a residual solution of not less than 0.005 molar H.sub.2 SO.sub.4.
U.S. Pat. No. 4,043,599 to Lingane et al. discloses an in-situ leaching process in which an oxidant containing acid lixiviant is preceded by an oxidant free-acid solution to minimize acid consumption during the leaching operation. Both the preinjected acid solution and the lixiviant may contain sulfuric acid. The acid preinjection step eliminates or minimizes a substantial rise in pH value in the deposit during the oxidative leaching process. The process is particularly applicable to the recovery of copper values from an ore having a low pyrite to chalcopyrite ratio but is also disclosed for use in pitch blend deposits.
In many in-situ leaching operations, particularly where the subterranean ore body has a high carbonate content, carbonate lixiviants are employed. These lixiviants contain carbonate or bicarbonate ions or mixtures thereof which function to complex the uranium in the form of water-soluble uranyl carbonate ions. The carbonate lixiviants may be formulated by the addition of alkali metal or ammonium carbonates and/or bicarbonates or by the addition of carbon dioxide together, in most cases, with an alkaline agent, e.g. sodium hydroxide or ammonia, to control the pH. An in-situ leaching procedure in which the lixiviant contains carbonate and bicarbonate ions is disclosed in U.S. Pat. No. 4,105,253 to Showalter. In this process the injected lixiviant is a neutral or near-neutral solution of carbonic acid formulated by dissolving carbon dioxide in the injected water. The pH of the injected lixiviant is maintained within the range of 6 to 7 to provide a bicarbonate ion concentration between about 380 ppm and 1,000 ppm.
U.S. patent application Ser. No. 934,933 filed Aug. 18, 1978 by Edward Thomas Habib, Jr. discloses an in-situ leaching process which involves the injection of a near-neutral or alkaline lixiviant in which an alkali metal sulfate such as sodium sulfate is employed as a leaching agent. The pH of the lixiviant may range down to about 6 and the lixiviant may also containing minor amounts of alkali metal carbonates or bicarbonates.